(1) Field of the Invention
The present invention relates to a structure of a chamber for a combustion engine.
(2) Description of Related Art
As shown in FIG. 9, conventional internal combustion engine 200 includes a cylinder head 203, a piston 201 and a chamber 205 which is formed from a bottom surface 204 of cylinder head 203 and a top surface 202 of the piston 201.
In the engine 200, squish areas 206 and 207, which are narrow spaces formed between the piston top surface 202 and a bottom edge 205A of the chamber 205, are made when the piston 201 is in Top Dead Center (TDC).
Accordingly, in the engine 200, it is possible to generate squish currents (see arrow “SF” in FIG. 9), which are air currents individually spurted out from the squish spaces 206 and 207 to the center of chamber 205 when the piston 201 comes up from Bottom Dead Center (BDC) to TDC. Further, in FIG. 9, each of ‘h206’ and ‘h207’ indicates the height of squish spaces 206, 207, respectively.
Besides, as shown in FIG. 10, a valve recess 210 is formed on the top surface 202 of the piston 201 so that intake valves and exhaust valves (not shown in FIGS. 9 and 10) do not hit against the top surface 202 partly because a Variable Valve Timing (VVT) mechanism (not shown in FIGS. 9 and 10) is equipped with the engine 200.
In other words, a valve stroke of the intake and/or the exhaust valve in an engine having the VVT mechanism is longer when the piston 201 is at TDC as compared to an engine without the mechanism. Accordingly the valve recess 210 is formed on the piston top surface 202 to avoid the valves hitting against the piston 201.
For example, a laid-open publication Japanese Utility Model (Kokoku) HEI 7-36107 discloses techniques related with the valve recess.
However, as shown in FIG. 10, it is necessary to move a top ring groove 208B, down (toward a piston pin 211 shown in FIG. 9) in which a piston ring (top ring) is fitted, when a valve recess is formed as deeply as the valve recess 210 shown in FIG. 10 because it is necessary to keep a thickness between the valve recess 210 and the top ring groove 208B as indicated ‘h205’ in FIG. 10.
Accordingly, it is impossible to avoid increasing the height of a top and 209, which is formed around the side surface of the piston 201 and is formed between the top ring groove 208B and the top surface 202, to keep necessary distance between the top ring groove 208B and the top surface 202.
In FIG. 10, an imaginary top ring groove 208A, formed when the valve recess 210 is not formed on the top surface 202, is shown by a dashed line.
Further, ‘h209A’ shows the height of the top land 209 when the valve recess 210 is not formed on the top surface 202. Alternatively, ‘h209B’ shows the height of the top land 209 when the valve recess 210 is formed on the top surface 202.
In other words, FIG. 10 shows that it is impossible to avoid increasing the top land's height due to forming the valve recess 210 by comparing between the height h209A and h209B.
However, increasing the volume of a quenching area which is a space between the top land 209 and a cylinder liner (not shown in FIGS. 9 and 10) is inevitable, if the height of the top land 209 is increased. Namely, flames from combustion in the chamber 205 cannot spread into the quenching area, and thus quality of the exhaust gas emitted from the engine 200 gets worse due to increasing amounts of HC (Hydro Carbons as unburned fuel).
In order to avoid worsening of the quality of exhaust gas, the location of the top surface 202 may be lowered toward the piston pin 211, i.e., the height h211 between the top surface 202 and the center C.sub.201 of the piston pin 211 is reduced.
But, it is also inevitable to increase the heights h206 and h207 of the squish areas 206 and 207. Accordingly, the fuel consumption becomes bad due to difficulties in generating adequate squish currents SF.